A ribbon of optical fibers comprises a set of optical fibers placed parallel to one another and embedded in a common outer coating, referred to as the “matrix”, e.g. made of a resin, the coating presenting a flat outer profile. The number of fibers grouped together in a ribbon may lie in the range 4 to 24, each fiber comprising in conventional manner a silica core of diameter less than 10 micrometers (μm), silicon cladding around the core with a diameter of about 125 μm, and a protective sheath of resin having a diameter equal to about 250 μm. FIG. 1 is thus a diagrammatic section of a ribbon 2 comprising a plurality of optical fibers 19, the ribbon 2 itself being coated in a matrix 20.
In optical fiber telecommunications networks where such ribbons are used, it is very often useful to be able to connect all or some of the fibers of a ribbon from which the coating has been removed to all or some of the fibers of another ribbon from which the coating has likewise been removed. Such a connection is made by means of a splice. The splice is made after stripping the sheaths from the individual fibers and putting the fibers that are to be connected together into end-to-end contact, the connection being held together by mechanical means, by welding methods, or by adhesive. After connection, the splice is covered in protection that gives rise to considerable extra thickness compared with the ribbon.
In order to make connection easier, it is important to have an extra length of ribbon available so as to be able to bring the end of the ribbon to a connection tool. This extra length makes it easier to handle the ends of the two ribbons without subjecting the ribbons to excessive stress, and in particular without either of the ribbons being subjected to bending exceeding a predetermined limit beyond which bending can lead to light being lost by attenuation, to damage, or even to the ribbon breaking. Once the connection has been made, the extra length of ribbon including the splice with its protection is placed inside a stowage cassette. Several types of stowage cassette have already been disclosed.
Thus, document FR 2 748 576 describes a cassette for coiling optical fiber ribbons carrying splice protection by using a cylindrical hub that is rotatable about the center of the cassette. The fact that the hub turns makes it easier to wind the ribbon around the hub; each of the ribbons connected together by a splice enters via a respective different entry to the cassette and the splice protection is fixed in a cavity formed in the thickness of the hub. That technique enables the ribbon to be held against the hub and prevents the ribbon from escaping from the cassette.
Nevertheless, implementation raises certain difficulties. In addition to the fact that the mechanical device is itself quite complicated, it is also necessary, for proper operation, to ensure that both ribbons connected together by a splice present the same length from their respective entry points into the device. Otherwise, there will always be a length of ribbon that is not properly stowed and that will give rise to installation stresses which must be overcome by a person acting on the telecommunications network.
Document WO 94/27176 describes a cassette for coiling optical fiber ribbons carrying a splice with protection. That cassette uses means for guiding the ribbon inside the cassette; the ribbons connected together by a splice enter the cassette via respective different entries and in opposite directions, and the splice protection is secured by a holding device. The ribbon is thus stowed by following guide means and by fixing the splice protection.
However, the use of such guide means distributed over the surface of the cassette requires the cassette to have a large stowage area in order to ensure that the ribbon has a radius of curvature that is large enough, particularly when the ribbon is long. The act of winding the ribbon into the cassette from the outside through the guide means causes the radius of curvature to decrease quickly as the ribbon comes closer to the center of the cassette.
In addition, securing the splice protection gives rise to significant installation stresses that are often associated with a spring effect of the ribbon, which can no longer take up its natural position. Such stresses can lead to undesirable twisting, and to radii of curvature of the ribbon that are not under control and that lead to light being lost by attenuation.